Orchids or dandelions? … four fifths of children can thrive in most environments.
Photograph: Barry Batchelor/PA

Some people seem to have terrible childhoods and yet manage to thrive despite them. Others grow up in loving homes but suffer from mental and physical health difficulties, even if their siblings do not. Why?

Research shows that about 15 to 20 % of children experience well over half of the recorded psychological illnesses. The remaining 75 to 80 % are comparatively healthy. This pattern continues into adulthood, and appears to be true for children around the world.

The first variable you would expect to account for these differences is whether the child comes from a prosperous or a poor background. It’s true, children from poorer backgrounds suffer slightly more illnesses and show more signs of psychological disturbance, but socioeconomic factors by no means account for all children who are in that fifth of the population who suffer half the illnesses. Whether a child blossoms or falters is driven wholly neither by environment nor by genetics, but by the interplay between the two. In this book, paediatric health expert W Thomas Boyce identifies two personality types. He argues that four fifths of children appear to be “dandelions”, who can thrive in most environments. The remaining fifth are “orchids”, who are more exquisite and unusual and have a higher potential than dandelions – but for this to be realised they require a particular environment and careful gardening. Like delicate plants, these children, if dealt with insensitively, have a greater tendency to run into problems.

How do you tell if a child is an orchid? They tend to be sensitive, shy, have negative emotional reactions to novel or changing conditions, and perhaps display challenging behaviour. But these symptoms cannot be taken as proof that a child will necessarily respond in a certain way to tests and show a high stress biological reaction to an external stressor. The symptoms are only correlations, they can only indicate that it is likely.

Boyce and his team test the biological stress response in children by setting various tasks such as watching an emotional video, having to repeat a string of numbers back to the researcher and having a drop of lemon juice put on the tongue and being asked to say what that was like. Although the experiment is scripted, and the researchers who test the children are all warm towards their subjects, how much cortisol (a stress hormone) and how much their autonomic nervous system (flight-or-fight response) is stimulated varies enormously, though in a predictable way. Time and time again, about four fifths of subjects show low levels of biological stress in these tests, and a fifth exhibit significantly higher stress levels. The children who show the higher stress response often have a slightly warmer right ear when their temperatures are taken, while others often have a slightly warmer left ear. Again, these differences in ear temperature can only indicate a possibility, not a certainty.

When children are tested after a stressful time, such as a family breakup, an earthquake or a different change in environment a pattern emerges. If orchids get the right nurturing, sufficient soothing and opportunities for self-expression – in other words, an environment that allows their sensitivities to work for them – they come out on top, higher than the dandelions. But if their environment works against them, they sink to the bottom, below the more robust dandelions who are less affected by their environment. The take-home message of the book is: orchid children are more susceptible to both negative and positive social conditioning; they have both the best outcomes and the worst.

Boyce tells the story of himself and his sister: he the dandelion, she the orchid. Not being so susceptible to the sometimes critical atmosphere of their childhood home, he went on to excel, whereas her early promise was confounded by physical and mental illnesses – she killed herself when she was 53. With hindsight, he can see that he was only slightly troubled by, say, his parents fighting, whereas she was immobilised with fear, frozen and traumatised by it. He believes a more sensitive, nurturing environment would have allowed her confidence and obvious talents to blossom and her story to have had a happier outcome.

This is a necessary and important book. To know that one fifth of people do not have a choice about how they physically react to stress should make us more understanding of the differences between us all. Children should be nurtured so that both orchids and dandelions can thrive. But I worry about how the orchid and dandelion theory might be employed. The danger of putting people into categories is that we unwittingly respond not to the person, but to their label.

• Phillippa Perry’s The Book You Wish Your Parents Had Read will be published by Penguin Life in March. The Orchid and the Dandelion by W Thomas Boyce is published by Bluebird (RRP £20). To order a copy for £13.99, go to guardianbookshop.com or call 0330 333 6846. Free UK p&p over £15, online orders only. Phone orders min. p&p of £1.99.